E-500 problem...

I just bought a used E-500 and have been playing around with it today. Seems odd to purchase such a camera these days but I owned one of these around 8 years ago and I decided to buy one from ebay for some nostalgia and to use as a no nonsense DSLR that I can play with.

Works great and seems spot on except for one thing... There is a line across every image I take...

Taken at 200iso with the lens cap on. Feint line slightly to the right of centre

Taken at 1600iso with the lens cap on. Much more obvious line.

As you can see, the line is much more obvious, the higher the ISO, but is visible in darker areas at all ISO. It's barely noticeable at 100ISO but is still there if you look for it.

I did a test and took a picture with the lens cap on at 1600iso but an exposure length of 10 seconds so that the dark frame subtraction filter kicked it. The image was clean as a whistle. No line at all. This leads me to believe that it is some kind of hotpixel problem on the sensor? Maybe a row of over sensitive or burnt out pixels? I don't really know how these things work from that point of view.

I've tried various settings on the camera and no matter what, the line is always there. I've tried pixel mapping too which made no difference. Is there a service menu I can access that will run further sensor mapping like on some pentax cameras?

It's something I could possibly live with if noise filters would remove or hide it somewhat but unfortunately, nothing I've tried seems to have any effect and I'm not prepared to clone it out of every image I take... Is there anything anyone can suggest, bar returning the camera for a refund?